Many structures, such as vehicles vibrate during operation. Further, each structure has a natural or resonant frequency that depends upon its shape or configuration. Structures of various types are frequently designed so that operational vibration does not match the natural frequency of the structure, so as to avoid natural or resonant frequencies. This is done to prevent damage to the structure, since a resonant frequency can focus mechanical energy into a specific location or part of the structure, and exceed the material strength of the structure at that location, potentially causing damage or failure. Exemplary materials used to fabricate parts include, but are not limited to, materials used to fabricate the mast, transmission mounts, landing gears, etc.
Vibrational forces may be caused by motions of engines, electric motors, etc. and even sound waves. While vibrations occasionally may have desirable consequences, in most arenas, it is desirable to counter or substantially eliminate most vibrational forces, or otherwise redirect a material's load path to preserve the structural integrity of a material and/or structure.
One category of vehicles that are affected by this issue is aircraft, such as rotary wing aircraft (e.g. rotorcraft such as helicopters, etc.). Rotorcraft are frequently subject to high vibration environments. Levels of vibration vary based on factors such as the rotor speed(s), environmental factors and payload. In the field of rotorcraft, additional vibrational forces may be induced to rotorcraft via the rotational operation of rotors. Left unchecked, vibrations in rotorcraft, or other large mobile or stationary structures can accelerate structural fatigue in the materials and components comprising the aircraft. For example, the natural frequency of a rotorcraft airframe can be excited by various factors occurring during a flight cycle including but not limited to: landing, taking off, bump, shifting center of gravity of the rotor, etc. If the natural frequency of the airframe materials is close to the rate of rotation of the rotor, the vibration can be amplified, and vibration increases to a level exceeding the strength of materials used to fabricate parts including, but not limited to, the mast, transmission mounts, landing gears, etc.
Vibration levels, in turn, often determine or limit the size and weight of a given rotary wing aircraft, or limit rotor speeds during operation. To address these issues, existing rotorcraft airframes are frequently stiffened by adding mass to the structure in order to avoid undesirable (e.g. resonant) frequencies of vibration. In the case of rotorcraft, the added mass can be significant (e.g. up to or exceeding hundreds of pounds). Improved load-bearing materials can be helpful in addressing this issue, but generally do not, on their own, allow for a reduction in vibration tuning mass. Furthermore, adding parasitic weight can restrict the speed and payload capabilities of the aircraft, as it is generally considered more desirable to have a lighter weight aircraft to allow more payload and/or achieve higher fuel efficiency.
In addition to the selective addition of weight, another approach for avoiding natural or resonant frequencies is to only operate a vehicle within certain operational ranges, so that only certain vibrational frequencies are possible. However, this approach limits the utility of the vehicle.
The present application relates to one or more of the above issues. Apparatuses, systems and methods for avoiding a structure's critical frequency on rotorcrafts that do not appreciably add to the overweight of the rotorcraft, or that would allow design of a rotorcraft to be made independent of the consideration of vibrational forces would be advantageous.